1. Technical Field
The present disclosure relates to an apparatus for detecting the position of a rotor of an electric motor, in particular a brushless motor, and to the related method.
The present disclosure permits to optimize the procedure of detecting or identifying the position of the rotor, in particular in sensorless brushless motors typically used as spindle motors in hard disks, CDs, DVDs, etc.
2. Description of the Related Art
The identification procedure, normally called “inductive sense”, takes advantage of the different response of the current in the motor winding with respect to a voltage pulse applied to the ends of the windings of the motor itself. The knowledge of the rotor position permits to optimize the motor startup procedure and is therefore a very important factor. The procedures of identifying the rotor position should therefore be characterized by high performances in terms of precision and insensitivity to disturbances.
Indeed an error in detecting the rotor position results in an imprecise excitation sequence of the stator phases and in a consequent efficiency reduction or, in the worst case, in a failure of the motor startup procedure (loss of synchronism).
The methods of detecting the rotor position (inductive sense) of known art only ensure good performances under conditions of a stopped rotor or while rotating at very low speed, i.e., under all those conditions in which the effect of the back electromotive force (BEMF) may be considered negligible.
Many startup techniques exist, some completely executed in open loop (startup in open loop) while in others, a detection of the rotor is indirectly used for generating the driving sequence, i.e., in closed loop (startup in closed loop).
The methods commonly used for detecting the position of the rotor at very low speeds (or stopped) are based on the analysis of the current in the windings upon a voltage pulse applied to the windings themselves. The principle is based on the phenomenon of magnetic saturation which is affected by the position of the permanent rotor magnet and which modifies in turn the profile of the current. Therefore, by analyzing the current, i.e., by analyzing the rise time Tr or the fall time Tf upon the application of a voltage step, the position of the rotor may be unambiguously determined, and therefore the stator phases may be excited to generate a torque of suitable value. The sequence of “identification step” (inductive sense) and “excitement step” (torque generation) permits to take the rotor to a speed such as to make the back electromotive force (BEMF) detectable. When BEMF reaches sufficiently high values, the startup procedure is considered concluded. Since then, the rotor position is extrapolated by directly analyzing the BEMF signal and no longer from the inductive sense procedure.
The inductive sense procedures are based on a comparison of certain parameters, typically the duration to reach a fixed current threshold, i.e., the rise time Tr, or the duration required to discharge the current circulating in the motor windings, i.e., the fall time Tf. In these cases the result is affected by the errors made in the individual measurements, since it is based on the comparison of various measurements made in sequence.
U.S. Pat. No. 6,841,903 describes a method for detecting the position of a rotor in a DC motor having N phases and a plurality of windings, comprising the steps of: connecting at least two of the windings between first and second prefixed voltages according to a first current path over a prefixed time; permitting the current stored in the two windings to be discharged by means of a second current path; comparing the voltage at the ends of one of the two windings with a third prefixed voltage and supplying a control signal when the voltage has an absolute value lower than a third prefixed voltage; performing the above-indicated steps for each of the pairs of motor windings; detecting the rotor position according to the control signals obtained.
The typical problem of an inductive sense procedure is that of the precision in the case wherein the motor moves, i.e., when the back electromotive force (BEMF) is not null or at least negligible. Under these conditions, the current pulses are modified by the presence of BEMF, thus generating an error when detecting the rotor position. Therefore, in the known art systems, a maximum limit of BEMF (and therefore of speed) exists, which may not be overcome if the desired precision and reliability in detecting the position of the rotor is to be ensured.